Measuring Current Using Digital Multimeters

Measuring Current Using Digital Multimeters

Introduction

Current is the rate at which charge passes a certain point in a circuit. Current not only has a value—it also has a direction. The link to the right provides more information about current.

The direction of the current must be accounted for when making a measurement; when you connect your DMM to a circuit, you will be assuming a particular current direction. The sign of the reading displayed on the DMM reflects this assumption—if the actual and assumed directions are the same, the displayed number will be positive. Conversely, if the number displayed is negative, the actual direction is opposite to your assumption.

Note

The way in which the probes are connected to your circuit defines the assumed direction of the measured current: The assumed current direction is such that positive current enters the A (or µA mA) port and leaves the COM port.

Current measurements tend to be viewed as being more difficult than either voltage or resistance measurements. This is typically because, when measuring current, you have to make sure that all of the current which you want to measure passes through the DMM. This invariably requires that you break your circuit apart at certain points and then insert the DMM at the two ends of those breaks. Of course, after measuring the current, you will also need to re-connect the circuit to its initial condition.

The other complication of measuring current is that most DMMs have multiple ports for current measurement—our DMM has two; they are labeled A and µA mA. The reason for the two ports is because the current through the DMM has to pass through a fuse. If too much current passes through the fuse, it will blow out. Unfortunately the construction of the DMM is such that, if you use a high capacity fuse in the measurement of a small current, you probably won't be able to read the current value accurately1.

DMM Settings

There are two possible DMM configurations which we can use when measuring currents:

  1. When measuring large currents, plug one of the DMM leads into the port with the A designation. The other DMM lead is plugged into the COM port. In this case, the DMM dial should be turned to the A setting. This configuration is shown in Fig. 1(a).
  2. When measuring small currents, plug one of the DMM leads into the port with the µA mA designation. The other DMM lead is plugged into the COM port. In this case, the DMM dial should be turned to either the mA setting (if the current is in the milliamp, or thousandths of an amp range) OR the µA setting (if the current is in the microamp, or millionths of an amp range). This configuration is shown in Fig. 1(b).
Figure 1. DMM configuration for current measurement.

Probe Connections

Now we have to connect the DMM leads to our circuit. As indicated above, this usually involves disconnecting parts of our circuit so that we can connect the DMM appropriately. For example, suppose we want to measure the current I between components 1 and 2 in the circuit from Fig. 2. In order to measure this current, we will need to break the circuit apart between the two components, as shown in Fig. 3(a)2.

Now we place our DMM between the two components, as shown in Fig. 3(b); current will flow through component 1, through the DMM, and then through component 2. At this point, we also need to make sure that our measurement matches the sign of the current we want to measure. Since the assumed direction of the current I is from component 1 to component 2, we need to set up our DMM so that the current enters the A (or µA mA) port and leaves the COM port3.

Figure 2. Current I to be measured.
Figure 3. Placement of an ammeter for current measurement.

One final comment on current measurement: as we pointed out above, we have two different ports on the DMM with which we can make current measurements. If we try to measure a high current with the low-current port, we can burn out the fuse. It is always possible to replace a burnt fuse; however, it is inconvenient and an easily avoidable situation. The question is, how do we know what the current is before we measure it4? The answer is, we don't. Therefore, to play it safe, always make sure your current measurement is using the high-current port first. If that measurement indicates a low current, then switch to the lower-current port.

Important Points

When measuring current using a DMM:

  • Always make a preliminary measurement using the highest-current port; if no current registers when using this port, then switch to the lower-current port. Chances are good that you will ignore this advice at some point as you become more confident in wiring up your circuits. (Replacement fuses, by the way, are available at most electronics stores.)
  • The DMM function selection should be set at A, mA, or µA.
  • The A or µA mA and COM ports are used.
  • Positive current enters the A or µA mA port and leaves the COM port. Current entering the COM port and leaving the A or µA mA port will be displayed as a negative value.
  • When measuring current, the circuit must be “broken” and the DMM inserted in the break so that all of the current to be measured passes through the DMM.
  • The A port is used to measure higher currents, and the µA mA port is used to measure low currents.

Test your Knowledge!

  1. We want to measure the current I1 with the indicated direction of the circuits below. Which of the diagrams shows the correct measurement?
  2. The actual current through the circuit elements are shown on the left of each figure. The DMM connections used to measure these voltages are shown on the right of each figure. What is the displayed voltage for each case?

Selected Answers

  1. (b)
  2. a) -3A (negative)

    b) 2A (positive)

    c) -4A (negative)


  • 1You will often just get a reading of zero amps. We say that the measurement has low sensitivity; by which we mean that we can't really tell the difference between two small numbers (.005 amps is basically the same as 0.006 amps, and both are pretty much the same as 0A). To increase the sensitivity, we need to use a fuse that will burn out at higher currents. So we can get either good sensitivity or we can measure large currents—we don't get the luxury of both at once!
  • 2For the circuit of 3(a), there will no longer be any current flowing between the two components. That situation will be fixed once we connect our ammeter.
  • 3If, when we make our measurement, current actually goes in the opposite direction (from component 1 to component 2) our measured current will be negative. The important thing at this stage is to keep track of what direction corresponds to positive current—not trying to set up our measurement so that we always get a positive value.
  • 4A little later, we will learn how to calculate the currents (and voltages) in our circuits, but what if we make a mistake in our calculations? Or what if we wire up our circuit wrong?

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